Heat spreader and method of making the same

ABSTRACT

A heat spreader having at least two adjoining strips of pyrolytic graphite material is made by cutting a strip from a sheet of pyrolytic graphite in the z direction. Thermal conductivity in the xy plane of the graphite sheet is greater than in the z direction. The z direction cut provides strips which are then each individually oriented 90 degrees such that the thickness direction of the original pyrolytic graphite sheet becomes the width or length of the cut strip. A face on the side of a first strip adjoins a face on the side of a second strip. Due to the greater thermal conductivity in the xy plane of the strips as compared to in the z direction heat transfers more rapidly in the length and thickness direction of the strips than across adjoining sides of the oriented strips.

BACKGROUND

The present invention relates to a heat spreader for conducting heatfrom a device and a method of making the heat spreader. Electroniccomponents are becoming smaller while heat dissipation requirements arebecoming greater. In order to dissipate heat generated by theseelectronic components, heat spreaders are utilized between theelectronic component and a heat sink. Heat spreaders can be made of asolid thermally conductive metal. The solid conductive metal has alimited ability to spread heat and has limited thermal conductivitycharacteristics.

SUMMARY

According to the present invention, a heat spreader and a method formaking the heat spreader is provided, and a method of dissipating from aheat source are disclosed.

In some embodiments, a heat spreader is provided which has at least twoadjoining planar elements or strips of pyrolytic graphite material. Thestrips are made by cutting strips from a sheet of pyrolytic graphitesuch that the sheet has a cut there through in the z direction. Thermalconductivity in the xy plane of the pyrolytic graphite sheet is greaterthan in the z direction. The z direction cut provides strips which arethen each individually oriented about 90 degrees such that the thicknessdirection of the original pyrolytic graphite sheet becomes the width orlength of the cut strip. A portion of a lateral side of a first stripwhich has been formed by cutting the sheet of graphite and orientedabout 90 degrees adjoins a face on the side of a second strip. Due tothe greater thermal conductivity in the xy plane of the strips ascompared to in the z direction heat transfers more rapidly along thelength of the strip and in the thickness direction of the orientedstrips than across a side of a strip which adjoins an adjoining strip.

In some embodiments of the invention the side of a first strip whichadjoins the side of a second strip is coextensive with the second side.

In some embodiments of the invention three or more strips ofsubstantially equal length are placed side by side.

Another embodiment of the invention is a method of making a heatspreader by providing at least two pyrolytic graphite strips or planarelements from a sheet of pyrolytic graphite. A cut is made in thethickness direction of the sheet which is known as the z direction. Thethermal conductivity of the sheet in the z direction or as is commonlyreferred to as the c direction is relatively low as compared to thethermal conductivity in the xy plane or as is commonly referred to asthe a directions or axes. The side of a first strip is then placed suchthat the side adjoins the side of a second strip. In this configurationheat transfers more rapidly along the length of the strip and in thethickness direction of the oriented strips than across a side of thestrip which adjoins an adjacent strip.

Another embodiment of the invention is a method of placing the heatspreader in a heat conducting relationship with a heat source byproviding adjoining pyrolytic graphite strips. The side of a first stripis placed such that the side adjoins the side of a second strip. Heattransfers more rapidly along the length of the strip and in thethickness direction of the oriented strips than across a side of thestrip which adjoins an adjacent strip. Heat is conducted from the heatsource into the first strip and second strip. Heat is conducted throughthe heat spreader in the direction of the a directions or axes of thepyrolytic graphite strips.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a parallel perspective view of a sheet of pyrolytic graphitefor use in the present invention showing the direction of the a and caxes of the layers of pyrolytic graphite of the sheet;

FIG. 2 is a parallel perspective view of the sheet of pyrolytic graphiteof FIG. 1 showing a first planar element which has been diced from thesheet and separated therefrom;

FIG. 3 a shows the first planar element of FIG. 2 after orientation ofabout 90 degrees;

FIG. 3 b shows the first planar element and the second planar elementprior to adjoining;

FIG. 4 shows an embodiment of the heat spreader of the present inventionand the direction of the a and c axes of the pyrolytic graphite in thefirst planar element and second planar element;

FIG. 5 shows another embodiment of the heat spreader of the presentinvention and the direction of the a and c axes of the pyrolyticgraphite in the first planar element and second planar element;

FIG. 6 shows another embodiment of the heat spreader of the presentinvention and the direction of the a and c axes of the pyrolyticgraphite in the first planar element, second planar element and thirdplanar element;

FIG. 6A shows a third planar element of the heat spreader of FIG. 6;

FIG. 7 shows the heat spreader of the present invention in combinationwith an electronic device and a heat sink; and

FIG. 8 shows another embodiment of the heat spreader of the presentinvention having a throughhole in the thickness direction of the planarelements of the heat spreader.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in detail by reference to thefollowing specification and non-limiting examples.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following embodiments are, therefore, to beconstrued as merely illustrative, and not limitative of the remainder ofthe disclosure in any way whatsoever.

Graphite is made up of layer planes of hexagonal arrays or networks ofcarbon atoms. These layer planes of hexagonal arranged carbon atoms aresubstantially flat and are oriented so as to be substantially paralleland equidistant to one another. The substantially flat parallel layersof carbon atoms are referred to as basal planes and are linked or bondedtogether in groups arranged in crystallites. Conventional orelectrolytic graphite has a random order to the crystallites. Highlyordered graphite has a high degree of preferred crystallite orientation.Accordingly, graphite may be characterized as laminated structures ofcarbon having two principal axes, the “c” axis or direction which isgenerally identified as the axis or direction perpendicular to thecarbon layers and the “a” axes or directions parallel to the carbonlayers and transverse to the c axes.

Referring now to the drawings in detail, wherein like reference numeralsindicate like elements through the several views, there is shown in FIG.1 a sheet 10 for making the heat spreader of the present inventionhaving axes a which are in the direction of the hexagonal array ofcarbon atoms. The c axis as shown is perpendicular to the carbon layers.

Graphite materials that exhibit a high degree of orientation includenatural graphite and synthetic or pyrolytic graphite. Natural graphiteis commercially available in the form of flakes (platelets) or as apowder. Pyrolytic graphite is produced by the pyrolysis of acarbonaceous gas on a suitable substrate at elevated temperature.Briefly, the pyrolytic deposition process may be carried out in a heatedfurnace and at a suitable pressure, wherein a hydrocarbon gas such asmethane, natural gas, acetylene etc. is introduced into the heatedfurnace and is thermally decomposed at the surface of a substrate ofsuitable composition such as graphite having any desirable shape. Thesubstrate may be removed or separated from the pyrolytic graphite. Thepyrolytic graphite may then be further subjected to thermal annealing athigh temperatures to form a highly oriented pyrolytic graphite commonlyreferred to as HOPG.

In FIG. 2 is shown a sheet 10 of pyrolytic graphite having the directionof the a axes and the c axis as shown. A first planar element 12 orstrip is cut or diced from the sheet 10 of pyrolytic graphite and afterthe first planar element 12 is cut from the sheet the direction of the aaxes and c axis within the first planar element 12 remain in the samedirection as when the first planar element 12 formed part of the sheet10.

Planar element 12 after being cut from sheet 10 is oriented about 90degrees or about 270 degrees such that the direction of the c axis ofthe first planar element 12 changes from the direction shown in FIG. 2to the direction shown in FIG. 3 a. Accordingly, it can be seen thatafter orientation of the first planar element 12 the relative locationof first side 14 of first planar element 12 has changed from that shownin FIG. 2 to that shown in FIG. 3 a. A second planar element 16 as shownin FIG. 3 b is cut from sheet 10 and orientated 90 or 270 degrees in amanner similar to that described above for the first planar element 12.

According to an embodiment of the present invention, a first side 14 offirst planar element 12 which is out of the plane of the plane of theplanar element 12 is adjoined with second side 18 of second planarelement 16 which is out of the plane of the second planar element 16such that at least a portion the first side 14 adjoins at least aportion of the second side 18 as seen in FIG. 5.

In another embodiment of the invention, the first side 14 of the firstplanar element 12 can extend substantially coextensively with the secondside 18 of the second planar element 16.

As can be seen in FIGS. 4 and 5, the portion of the first side 14 of thefirst planar element 12 which adjoins the portion of the second planarelement 16 extends substantially normal to the first plane of the firstplanar element 12. The first plane of the first planar element 12 isdefined by the direction in which the major dimension h and minordimension g extend as shown in FIG. 4. The major dimension h and minordimension g can be of equal magnitude, however the major dimension h andminor dimension g are not the thickness dimension of the planar element.

The major dimension h and minor dimension g can be the first lateraldimension and second lateral dimensions of the first planar element 12.

The direction of the first lateral dimension or major direction h of theplanar element 12 and the thickness direction i of the first planarelement can be the direction of the a axes of the sheet 10 of pyrolyticgraphite from which the first planar element 12 is formed. The directionof the second lateral dimension can be the direction of the c axis ofthe sheet 10 of pyrolytic graphite from which the first planar element12 is formed as seen in FIG. 1. Therefore, as seen in FIG. 4, the firstplanar element has a relatively high thermal conductivity in the firstlateral dimension, here, major dimension h of the planar element and inthe thickness direction i of the first planar element but a relativelylow thermal conductivity in the second lateral dimension or minordimension g. Therefore, heat is conducted more readily along majordimension h and in the thickness direction i than in minor dimension gthan across first side 14 of first planar element 12 to second side 18of second planar element 16.

The heat spreader of the present invention can be made such that theplanar elements each have three sets of parallel sides. Each side can beorthogonal to two other sides of the planar element. The two oppositesides of a planar element can be spaced apart at substantially the samedistance along each side.

The sheets of pyrolytic graphite from which the planar elements are cutor diced by any means for cutting the sheets such as wirecuttingmachines, dicing machines, or slicing machines are available in sizeshaving a thickness in the f dimension shown in FIG. 1 of from 0.2millimeters up to 5 centimeters. A typical thickness is 1.3 centimeters.Commercially available pyrolytic graphite sheets are available having alength or d dimension of about 3 meters and the width dimension e can beas large as 40 centimeters. Pyrolytic graphite sheets suitable for usein the present invention are available from the Pyrogenies Group ofMinteq International Inc. of New York, N.Y. An example is PYROID® HTpyrolytic graphite.

In one embodiment the distance in which the first side and the secondside of the first planar element are spaced apart is at least about 1.5millimeters.

In another embodiment the distance at which the first side and thesecond side of the first planar element are spaced apart is from about1.5 millimeters to about 1.3 centimeters.

In another embodiment the distance at which the first side and thesecond side of the first planar element are spaced apart is from about1.3 centimeters to about 2.5 centimeters.

In another embodiment the distance at which the first side and thesecond side of the first planar element are spaced apart is at leastabout 1.3 centimeters.

In another embodiment the distance at which the first side and thesecond side of the first planar element are spaced apart is at leastabout 4.0 centimeters.

In another embodiment the distance at which the first side and thesecond side of the first planar element are spaced apart is from about1.3 centimeters to about 5.0 centimeters.

In another embodiment the distance at which the third side and thefourth side of the first planar element are spaced apart is at leastabout 1.0 centimeter.

In another embodiment the distance at which the third side and thefourth side of the first planar element are spaced apart is from about1.0 centimeters to about 40 centimeters.

The thermal conductivity of the sheets in the a axes of the sheets canbe from about 450 to about 2000 Watts/m° K. and the particular thermalconductivity for a particular application can be tailored. The thermalconductivity in the z direction or along the c axis can be as low asabout 2.0 Watts/m° K. or in the case of PYROID® HT pyrolytic graphite 7Watts/m° K. By comparison the thermal conductivity of copper is 400Watts/m° K. As copper has a density of 8.9 g/cc as compared to valuesfor pyrolytic graphite of as high as 2.25 g/cc, greater efficiencies andweight savings can be achieved using the heat spreader of the presentinvention.

Thermal grease can be used at the interface between the first planarelement 12 and second planar element 16. The heat spreader 22 of FIG. 4can be adjoined to a substrate such as a heat sink, here a copper plate20 as seen in FIG. 7 by any suitable means for adjoining the firstplanar element 12 and second planar element 16 to a substrate. In theevent that the heat spreader 22 is adjoined to a heat sink the means foradjoining the heat spreader 22 to the substrate permits the transfer ofheat from the heat spreader 22 to the substrate. A mechanical means suchas a clamping means can be a means to adjoin the heat spreader to asubstrate which in turn transfers heat from the heat spreader to a heatsink. Also, the heat spreader can be adjoined directly to a heat sink.Additional means for adjoining the heat spreader to a substrate or heatsink can be a bonding means. The bonding means can be a layer of metalor a layer which comprises metal on a planar element of the heatspreader which is bonded to the substrate such as by soldering at leasta portion of the metal containing layer to the substrate or heat sink.The layer is applied to a planar element on at least a portion of theplanar element which is to adjoin the substrate. After application ofthe metal containing layer on at least a portion of the planar element,the planar element can be adjoined to the substrate or heat sink bytechniques used in the semiconductor industry such as soldering or evenby a mechanical means such as a mechanical fastener.

Application of the metal containing layer on a portion of the planarelement which adjoins the substrate can be achieved by metallizationtechniques, sputtering or by applying a layer of solder to the portionof the planar element which is to be joined to the substrate. The planarelements can be provided with a surface treatment prior to theapplication of the metal containing layer using techniques suitable foruse on semiconductors.

Any means for joining the first planar element 12 and the second planarelement 16 can be used. For example, a mechanical clamping means such asa mechanical fastener can be used to join the first planar element 12and the second planar element 16 together or the first planar element 12and the second planar element 16 can be soldered together usingtechniques which are capable of joining carbon-based surfaces together.Upon adjoining of the first planar element 12 and the second planarelement 16 heat can transfer from the first planar element 12 and thesecond planar element 16 along the portion wherein the first planarelement 12 and the second planar element 16 are adjoined.

In another embodiment of the present invention, a heat spreader has afirst planar element 12, a second planar element 16 and a third planarelement 24 as seen in FIGS. 6 and 6A. The third planar element 24 is cutor diced from the sheet 10 of pyrolytic graphite and oriented in amanner similar to that in which the first planar element 12 and thesecond planar element 16 are cut. A third side 26 of second planarelement 16 is arranged such that the third side 26 adjoins a fourth side28 of third planar element 24. In a similar way, heat spreaders of thepresent invention can be made with a fourth, fifth or sixth etc. planarelement. Each additional planar element has a side which adjoins anadjacent side of a planar element of the heat spreader such that heattransfers more rapidly in the two dimensions of the heat spreader whichdo not have a side which adjoins an adjacent planar element.

Because the a and c axes of the pyrolytic graphite of all three of thestrips which make up this embodiment of the invention are arranged inthe direction shown in FIG. 6, heat is transferred more readily in the jand k dimensions as compared to the 1 dimension.

In FIG. 7 a heat spreader of the present invention is shown incombination with an electronic device 30 and a heat sink 20 which is acopper plate. Heat from the electronic device 30 is transferred to theheat spreader 22. From the heat spreader 22, heat is transferred mostrapidly in the direction of the thickness dimension i and in thedirection of lateral dimension h which are oriented in the a axes of thepyrolytic graphite from which the heat spreader 22 is made. Heat istransferred less rapidly across the interface between the first planarelement 12 and the second planar element 16.

The electronic device can be a microprocessor, an integrated circuit,high power devices such as laser diodes, LEDs, wide band gap, RF andmicrowave devices, power amplifiers, insulated gate bipolar transistors(IGBTs), application specific integrated circuits (ASICs), liquidcrystal display (LCDs) and other types of video displays.

In yet another embodiment of the present invention, at least one of thefirst planar element 12 and the second planar element 16 has athroughhole 32 at least partially therethrough. A core 34 of materialwhich can be isotropic or anisotropic such as pyrolytic graphite can beinserted into the throughhole 32. The core can be or can comprise ametal having a relatively high thermal conductivity or even diamond. Thecore 34 in the throughhole permits the transfer of more heat in thethickness direction i of the first planar element 12 or the secondplanar element 16.

The invention also includes another embodiment disclosing a method ofdissipating heat from a heat source comprising providing a heat spreaderhaving a first planar element and second planar element arranged asdescribed above. The heat spreader is placed in a heat conductingrelationship with a heat source such that the heat spreader conductsheat from the heat source into the first strip and second strip. Heat isconducted through the heat spreader in the direction of relatively highthermal conductivity.

Accordingly, it is understood that the above description of the presentinvention is susceptible to considerable modifications, changes andadaptations by those skilled in the art, and that such modifications,changes and adaptations are intended to be considered within the scopeof the present invention, which is set forth by the appended claims.

1. A heat spreader comprising: a) a first planar element of pyrolyticgraphite having a relatively high thermal conductivity in a direction ofa first lateral dimension of a first plane of the first planar elementand in the thickness direction of the first planar element and having arelatively low thermal conductivity in a direction of second lateraldimension of the first planar element, wherein the direction of thefirst lateral dimension of the first plane of the first planar elementand the thickness direction of the first planar element extendsubstantially in directions of orientation of a axes of the pyrolyticgraphite of the first planar element and the direction of the secondlateral dimension of the first planar element extends substantially inthe direction of the c axis of the pyrolytic graphite of the firstplanar element, b) a second planar element of pyrolytic graphite havinga relatively high thermal conductivity in a direction of a first lateraldimension of a second plane of the second planar element and in thethickness direction of the second planar element and having a relativelylow thermal conductivity in a direction of a second lateral dimension ofthe second planar element, wherein the direction of the first lateraldimension of the second plane of the second planar element and thethickness direction of the second planar element extend substantially indirections of orientation of a axes of the pyrolytic graphite of thesecond planar element and the direction of the second lateral dimensionof the second planar element extends substantially in the direction ofthe c axis of the pyrolytic graphite of the second planar element,wherein each of the first planar element and second planar element havea first side and a second side, the first side and second side of eachof the first planar element and second planar element beingsubstantially parallel and being spaced apart in the direction of thesecond lateral dimension at a first distance, the first side and secondside of each of the first planar element and second planar elementextending substantially normal to the direction of the second lateraldimension of the first planar element and the second planar elementrespectively wherein at least a portion of the first side of the firstplanar element adjoins at least a portion of the second side of thesecond planar element; and wherein the first side of the first planarelement substantially coextensively adjoins at least a portion of thesecond side of the second planar element.
 2. A heat spreader comprising:a) a first planar element of pyrolytic graphite having a relatively highthermal conductivity in a direction of a first lateral dimension of afirst plane of the first planar element and in the thickness directionof the first planar element and having a relatively low thermalconductivity in a direction of second lateral dimension of the firstplanar element, wherein the direction of the first lateral dimension ofthe first plane of the first planar element and the thickness directionof the first planar element extend substantially in directions oforientation of a axes of the pyrolytic graphite of the first planarelement and the direction of the second lateral dimension of the firstplanar element extends substantially in the direction of the c axis ofthe pyrolytic graphite of the first planar element, b) a second planarelement of pyrolytic graphite having a relatively high thermalconductivity in a direction of a first lateral dimension of a secondplane of the second planar element and in the thickness direction of thesecond planar element and having a relatively low thermal conductivityin a direction of a second lateral dimension of the second planarelement, wherein the direction of the first lateral dimension of thesecond plane of the second planar element and the thickness direction ofthe second planar element extend substantially in directions oforientation of a axes of the pyrolytic graphite of the second planarelement and the direction of the second lateral dimension of the secondplanar element extends substantially in the direction of the c axis ofthe pyrolytic graphite of the second planar element, wherein at least aportion of a first side of the first planar element which extends in adirection out of the first plane of the first planar element adjoins atleast a portion of a second side of the second planar element whichextends in a direction out of the second plane of the second planarelement, and a third planar element of pyrolytic graphite having arelatively high thermal conductivity in a direction of a first lateraldimension of a third plane of the third planar element and in thethickness direction of the third planar element and having a relativelylow thermal conductivity in a direction of a second lateral dimension ofthe third planar element, wherein the direction of the first lateraldimension of the third plane of the third planar element and thethickness direction of the third planar element extend substantially indirections of orientation of a axes of the pyrolytic graphite of thethird planar element and the direction of the second lateral dimensionof the third planar element extends substantially in the direction ofthe c axis of the pyrolytic graphite of the third planar element,wherein at least a portion of a third side of the third planar elementwhich extends in a direction out of the third plane of the third planarelement adjoins at least a portion of a fourth side of the second planarelement which extends in a direction out of the second plane of thesecond planar element.
 3. A heat spreader in combination with anelectronic device, wherein the electronic device is provided on the heatspreader and the heat spreader conducts heat from the electronic devicewherein the heat spreader comprises: a) a first planar element ofpyrolytic graphite having a relatively high thermal conductivity in adirection of a first lateral dimension of a first plane of the firstplanar element and in a thickness direction of the first planar elementand having a relatively low thermal conductivity in a direction ofsecond lateral dimension of the first planar element, and b) a secondplanar element of pyrolytic graphite having a relatively high thermalconductivity in a direction of a first lateral dimension of a secondplane of the second planar element and in a thickness direction of thesecond planar element and having a relatively low thermal conductivityin a direction of a second lateral dimension of the second planarelement, wherein at least a portion of a first side of the first planarelement which extends in a direction out of the first plane of the firstplanar element adjoins at least a portion of a second side of the secondplanar element which extends in a direction out of the second plane ofthe second planar element.
 4. A heat spreader in combination with anelectrical device and a heat sink, wherein the electronic device isprovided on the heat spreader and the heat spreader conducts heat fromthe electrical device to the heat sink wherein the heat spreadercomprises: a) a first planar element of pyrolytic graphite having arelatively high thermal conductivity in a direction of a first lateraldimension of a first plane of the first planar element and in athickness direction of the first planar element and having a relativelylow thermal conductivity in a direction of second lateral dimension ofthe first planar element, and b) a second planar element of pyrolyticgraphite having a relatively high thermal conductivity in a direction ofa first lateral dimension of a second plane of the second planar elementand in a thickness direction of the second planar element and having arelatively low thermal conductivity in a direction of a second lateraldimension of the second planar element, wherein at least a portion of afirst side of the first planar element which extends in a direction outof the first plane of the first planar element adjoins at least aportion of a second side of the second planar element which extends in adirection out of the second plane of the second planar element.
 5. Aheat spreader in combination with a heat sink wherein the heat spreaderis provided on the heat sink and wherein the heat spreader comprises: a)a first planar element of pyrolytic graphite having a relatively highthermal conductivity in a direction of a first lateral dimension of afirst plane of the first planar element and in a thickness direction ofthe first planar element and having a relatively low thermalconductivity in a direction of second lateral dimension of the firstplanar element, and b) a second planar element of pyrolytic graphitehaving a relatively high thermal conductivity in a direction of a firstlateral dimension of a second plane of the second planar element and ina thickness direction of the second planar element and having arelatively low thermal conductivity in a direction of a second lateraldimension of the second planar element, wherein at least a portion of afirst side of the first planar element which extends in a direction outof the first plane of the first planar element adjoins at least aportion of a second side of the second planar element which extends in adirection out of the second plane of the second planar element.
 6. Thecombination of claim 5 wherein the heat sink is a copper plate.
 7. Aheat spreader comprising: a) a first planar element of pyrolyticgraphite having a relatively high thermal conductivity in a direction ofa first lateral dimension of a first plane of the first planar elementand in a thickness direction of the first planar element and having arelatively low thermal conductivity in a direction of second lateraldimension of the first planar element, and b) a second planar element ofpyrolytic graphite having a relatively high thermal conductivity in adirection of a first lateral dimension of a second plane of the secondplanar element and in a thickness direction of the second planar elementand having a relatively low thermal conductivity in a direction of asecond lateral dimension of the second planar element, wherein at leasta portion of a first side of the first planar element which extends in adirection out of the first plane of the first planar element adjoins atleast a portion of a second side of the second planar element whichextends in a direction out of the second plane of the second planarelement, wherein at least one of the first planar element and secondplanar element has a throughhole at least partially therethrough and theheat spreader further comprises a core received in the throughhole thecore being of a material such that heat from a heat source can beconducted via the core into the thickness of the at least one of thefirst planar element and second planar element.
 8. A heat spreadercomprising: a) a first planar element of pyrolytic graphite having arelatively high thermal conductivity in a direction of a first lateraldimension of a first plane of the first planar element and in athickness direction of the first planar element and having a relativelylow thermal conductivity in a direction of second lateral dimension ofthe first planar element, and b) a second planar element of pyrolyticgraphite having a relatively high thermal conductivity in a direction ofa first lateral dimension of a second plane of the second planar elementand in a thickness direction of the second planar element and having arelatively low thermal conductivity in a direction of a second lateraldimension of the second planar element, wherein at least a portion of afirst side of the first planar element which extends in a direction outof the first plane of the first planar element adjoins at least aportion of a second side of the second planar element which extends in adirection out of the second plane of the second planar element, whereinat least one of the first planar element and second planar element has ahole at least partially therethrough and the heat spreader furthercomprises a core received in the throughhole the core being of ananisotropic material such that heat from a heat source can be conductedvia the core into the thickness of the at least one of the first planarelement and second planar element.
 9. The heat spreader of claim 8wherein the core comprises pyrolytic graphite.
 10. A method ofdissipating heat from a heat source, comprising: a) providing a heatspreader comprising: i) a first planar element of pyrolytic graphitehaving a relatively high thermal conductivity in a direction of a firstlateral dimension of a first plane of the first planar element and in athickness direction of the first planar element and having a relativelylow thermal conductivity in a direction of second lateral dimension ofthe first planar element, and ii) a second planar element of pyrolyticgraphite having a relatively high thermal conductivity in a direction ofa first lateral dimension of a second plane of the second planar elementand in a thickness direction of the second planar element and having arelatively low thermal conductivity in a direction of a second lateraldimension of the second planar element, wherein at least a portion of afirst side of the first planar element which extends in a direction outof the first plane of the first planar element adjoins at least aportion of a second side of the second planar element which extends in adirection out of the second plane of the second planar element; b)placing the heat spreader in heat conducting relationship with a heatsource; c) conducting heat from the heat source into the first strip andsecond strip; and d) conducting heat through the heat spreader in thedirection of relatively high thermal conductivity.